The Ofna .26 is a dog, very poor performance but it is reliable.
a .28 in a buggy is overkill, it is not going to be any faster. It will just wear out the drivetrain quicker and drink more fuel.
Find a nice .21 such as the Picco .21 BAS. Great little engine for 109 bucks.
There is about 2 minutes difference runtime between a .28 and a .21.
Buggies do not need a lot of bottom end torque. They need engines that can rev high to be fast. Almost all .21's turn more RPM's than the average .28.
The XTM 24.7 would be a good choice as well if you are on a budget.
If not drop a Jammin' .21, O.S. Vspec, GRP .21 or an Ex-Tech .21. Be prepared to go thru some tires with those engines! They are all wicked fast.